Instructions
Approximate the era or general look of the outfit. Does it remind you of a Jacqueline Kennedy look from the 1950s or early 1960s? Or does it appear to be a hippie or mod look from the late 1960s or the 1970s? Perhaps it reminds you of the 1980s or later. Mattel dresses Barbie in the current outfit of the day because she's a modern doll.
Look at pictures of Barbie outfits from that time period. Many websites offer free galleries to compare your vintage Barbie outfits to theirs. Take advantage of this opportunity, but remember that Barbies from different years dressed in the same outfits. For example, the 1959 Barbie wore a black-and-white-striped swimsuit, as did other Barbies from the 1960s.
Take the clothing to an experienced collector to narrow down the possible year your vintage Barbie outfit is from. You'll find this especially helpful if you own an outfit that Mattel repeated over the years. Find doll collectors in the Yellow Pages as well as on vintage doll websites. Some websites offer free appraisals if you email information and pictures.